Conventionally, a substrate glass for various displays, particularly, a substrate glass on which surface a metal or oxide thin film is formed, has been required to have the following characteristics:
(1) being substantially free of alkali metal ions, because when an alkali metal oxide is contained, alkali metal ions diffuse in the thin film to deteriorate film characteristics;
(2) having a high strain point so that deformation of a glass and shrinkage (thermal shrinkage) due to structure stabilization of the glass can be minimized when exposed to high temperature in a thin film formation step;
(3) having a low specific gravity for weight reduction, and being thin and flat;
(4) having sufficient chemical durability to various chemicals used in semiconductor formation, in particular, having durability to buffered hydrofluoric acid (hydrofluoric acid+ammonium fluoride; BHF) for etching SiOx or SiNx, a chemical solution containing hydrochloric acid used for etching of ITO, various acids (e.g., nitric acid, sulfuric acid) used for etching of a metal electrode, and an alkali of a resist removing liquid; and
(5) having no defect (bubbles, striae, inclusions, pits, flaws, etc.) in the inside and on the surface.
In a glass for substrates of displays, it is strictly required in particular to not contain bubbles, that is, to suppress air bubble generation in the production process or prevent generated air bubbles from remaining in a final product. As the refining agent added to a glass raw material of an alkali-free glass so as to remove bubbles, SnO2, F, Cl, SO3, etc. have been heretofore used.
SnO2 causes bubbles to grow by releasing O2 at a high temperature such as 1,500° C. or more and in turn encourages bubbles to float and burst at the melt surface; F and Cl allow the bubbles to expand in a reduced pressure state; and SO3 causes bubbles to grow by releasing SO2 and O2 in a melting oven.
Patent Document 1 describes a technique of using, as the refining agent of an alkali-free glass, any one or more of Sb2O3, SO3, Fe2O3 and SnO2 in combination with any one or more of F and Cl. Patent Document 2 describes that when SnO2 is used as the refining agent of an alkali-free glass, among others, S (sulfur) component is likely to remain as bubbles and therefore, it is preferable not to add S (sulfur) component. Patent Document 3 describes that when SnO2 is used as the refining agent of an alkali-free glass, if the amount of S (sulfur) charged is large, bubbles are rather increased due to re-foaming.